The field of the present invention particularly concerns small garden tractors which include hydrostatic transmissions for controlling the speed, direction and pulling power of the tractor. The transmission is provided with a rotatable control shaft which may be indirectly connected to a rocking foot pedal so that the control shaft is rotated in one direction when the pedal is rocked forward to effect forward motion of the tractor, and is rotated in the opposite direction when the pedal is rocked rearward for reverse motion. The speed of the tractor is proportional to the amount the foot pedal is depressed, while the engine is operated at a speed set by a conventional throttle device, the details of which do not directly affect the present invention.
A prior art control system of the type indicated is shown, for example, in the U.S. Pat. No. 3,505,896 issued to Phillips on Apr. 14, 1970, and assigned to the assignee of the present invention. For the operator's convenience when the transmission setting does not require changing for an extended period of time, the patented structure provides a temporary locking mechanism on the shaft to which the rocking foot pedal control for the transmission is attached. The locking mechanism comprises a second foot pedal with a grooved undersurface for receiving a sector of a disc connected to the shaft. The disc and groove are so shaped that they may be wedged together by forcefully depressing the locking foot pedal, thus allowing both the directional foot pedal and the locking foot pedals to be released while the transmission retains its set position. If the directional foot pedal is again actuated, the relative movement between the disc and groove causes them to disengage so that the locking mechanism is ineffective until it is again set by depressing the locking foot pedal.
It has been found in actual use that a locking device of the type described is not always effective for its intended purpose, in that the device sometimes disengages if the tractor is subjected to extreme jolts or vibration in operation. A further disadvantage is that, as a safety precaution, it is desirable that a transmission locking device be inoperative when the transmission is in a reverse driving mode, and this is not possible with the patented locking device. A feature lacking in the patented device and provided by the present system is that as a further safety precaution, it is desirable to have the locking device capable of being positively connected or disconnected to prevent its accidental use. The latter capability is also a convenience feature because of the relatively infrequent use of the locking device in many working conditions. In contrast to the relatively positive locking arrangement in the Phillips patent, supra, the present invention provides a brake assembly which will maintain the control shaft of the transmission in a selected position, but which can be manually overridden if the need arises by the foot pedal.